Since 1950s, scientists found that a phase velocity of Rayleigh wave in a layered medium is changed with frequency, and shows an obvious frequency dispersion property. Rayleigh wave in horizontally layered medium is generated by mutually interfering and overlapping P wave and shear wave at various interfaces in a seismic source region. The Rayleigh wave carries information of medium in various layers including P wave velocity, shear wave velocity, and density, and etc., and the velocities depend on a distribution of shear wave in the layered medium mainly. Variation of energy and velocity of Rayleigh wave induced by underground structure, frequency dispersion generated, which reflects some inherent characteristics of the layered medium. From this, deep underground structure can be solved by studying natural seismic wave's low dispersion curves of Rayleigh wave. Shallow underground structure such as engineering exploration, site evaluation and foundation treatment, obstacles and cavity detection, etc. can be solved by studying on higher dispersion curves by man-made seismic source.
Surface wave prospecting is the most popularly geophysical exploration method in engineering geophysical exploration, and can be divided into two types according to different seismic sources, that is, an active source method and a passive source method. Wherein, the active surface wave is generated by activating seismic source artificially. The passive source, as the name implies, is called with respect to the active source surface wave, that is, there is no need to generate seismic sources artificially and proactively, however, various vibrations generated by natural phenomenon such as tide, wind, volcano activity, etc., and by various human activities such as vehicle movement, mechanical movement in the factory, human movements, etc., are taken as the seismic sources instead.
In city engineering geophysical exploration, the active source surface wave geophysical exploration needs artificial seismic sources, and thus may have certain influence on the environment, and also have some requirements on site condition. Observation array needs to be linearly arranged, geophysical exploration can't be implemented frequently in complex regions of the city. Moreover, energy of surface wave activated by artificial seismic source is limited. The depth of exploration always fails to meet the need of engineering exploration. Thus, there are many limitations in the active source surface wave method in engineering exploration. For this reason, a passive source surface wave prospecting method is introduced into the field of engineering geophysical exploration.
One of the core problems of passive source surface wave geophysical exploration is about how to extract dispersion curves from collected seismic wave data. At the present stage, methods for extracting dispersion curves of passive surface waves mainly include: F-K (Frequency-Wavenumber) method and SPAC (Space Autocorrelation Method) based on Fourier transform. The both methods have own advantages and disadvantages respectively, wherein although the F-K method can identify higher modes of surface waves, it has a low resolution and data needs to be collected simultaneously with a great number of receiving points; since the SPAC method requires less receivers, it has now become the most popular passive source surface wave method. However, the SPAC method can't extract higher modes surface wave dispersion curves, but can only extract the fundamental mode dispersion curve, and the observation array still need to be arranged in certain rules and shapes.